Noctilucent clouds on the rise

Night-shining clouds have fascinated skywatchers for almost 140 years. Rob Banino finds out what they tell us about our changing atmosphere

- Rob Banino

The Krakatoa eruption of 1883 was one of the deadliest in recorded history. The volcano, which lies on an island in Indonesia's Sunda Strait, had been threatening to blow for months. It had been sending plumes of ash and steam into the sky since May of that year, but at 1pm on 26 August the pressure beneath its rocky cones finally became too much.

Four increasingly violent explosions over the next 24 hours all but destroyed the island. They killed over 36,000 people and could be heard 3,500 kilometres away in Australia. Twenty-one cubic kilometres of rock and ash was blasted across 800,000 square kilometres and over 80 kilometres up into the air.

So much ash was released into the atmosphere, the region was plunged into darkness for two and a half days. As the ash diffused and drifted around the world, its chemicals absorbed different wavelengths of light, causing spectacular red and orange sunsets and making the Moon glow blue for months.

There was enough ash lingering in the atmosphere a year later to cause summer temperatures in the Northern Hemisphere to drop by an average of 0.4°C.

Then, on a clear summer's night in 1885, amateur astronomers in the German town of Bad Kissingen spotted some new, mysterious-looking clouds. It wasn't so much that they were thinner and wispier than regular clouds, but they were visible after dark.

They actually appeared to be shining. 

Those mysterious clouds would become known as noctilucent, or night-shining, clouds (NLCs). They can be seen every year between May and August and have become a favourite target for many observers. And since that first sighting, we've gradually learned more about them.